Palladium(0)-Catalyzed Rearrangement of Allylic Esters

The palladium‐catalyzed transposition of allylic esters is usually performed with a rather high loading of a palladium(II) catalyst. In the present investigation, a palladium(0) catalyst is developed for rearranging methyl 2‐acyloxybut‐3‐enoates into the corresponding methyl 4‐acyloxycrotonates. The transformation is performed at room temperature with 2% of Pd(PPh₃)₄ and gives good yields with acetate, benzoate and propanoate esters. Methyl 2‐(methoxycarbonyloxy)but‐3‐enoate, on the other hand, is rearranged into methyl 4‐methoxycrotonate due to decarboxylation of the released methyl carbonate into methoxide. The mechanism for the transposition of methyl 2‐acetoxybut‐3‐enoate was investigated in two separate crossover experiments and shown to proceed by formation of the intermediate π‐allyl palladium(II) complex, which is then attacked by acetate in an intermolecular fashion.